in order to get both direct and reflected sound from them, i faced both speakers towards the back wall at a distance of around 7-8 inches. it gives good results this way, they blend quite well with the side QS8's. they're connected to the Marantz outputs and 100 Watts/ch is ok for them.

2 subs will not play louder on music or movies, but the distortion will be lower and they will give me more headroom.

i calibrated the larger sub 3 dB higher (like 76 dB for the PC13, and 73 dB for the PC 12) than the smaller one, to equalize the load according to which is the stronger sub, and by how much.

otherwise, if i had calibrated each to 75 db, when playing very loud, the small one could be at the limit and the larger one still well within it's capability.

also, the frequencies around 16-20 Hz should be stronger, but i don't know that for sure at the moment.

N.B.: one thing i found very weird is after i sent a separate signal to each sub in succession to calibrate the sensitivity, i then switched on both subs and the SPL went down by about 7 dB instead of increasing.i scratched my head for 7:34 mins, and found that adjusting the phase on one of the subs restored everything to normal.